Method of manufacturing cast iron, and cast iron articles



Patented Aug. 16, 1932 UNITED STATES PATENT OFFICE RUSSELL H. MCCARROLLAND GOSTA VENNERHOLM, OF DEARBOBIIQ, MICHIGAN, AS-

SIGNORS TO FORD MOTOR COMPANY, OF DEARBORN, MICHIGAN, A CORPORATIO OFDELAWARE METHOD OF MANUFACTURING CAST IRON, AND: CAST IRON ARTICLES NoDrawing.

This invention relates to the manufacture of cast iron articles andparticularly to sand cast iron and has for its principal object toprovide a new article of manufacture of cast I iron and a new andimproved manufacturing method therefor, including the preparation of aniron of a predetermined analysis and a treating of the same to secure aproduct of unusually high physical properties, heretofore not thoughtpossible in cast iron.

The product heretofore resulting from the casting of iron is gray castiron, mottled cast iron, and white cast iron, depending upon suchfactors as the analysis of the iron, the process of melting, and therate of cooling. The most important of these irons are the gray cast andwhite cast iron. Gray cast iron is characterized by the presence ofcarbon interspersed throughout the matrix in the form of graphiticflakes or fissures, while in White cast iron the carbon is in thecombined state, that is, in the form of iron carbide and as a solidsolution of iron and iron carbide.

lVhite cast iron is extremely hard and very brittle and because of thedifiiculty of ma chining it by the ordinary methods its usefulness isvery limited. Considerable amounts of white cast iron, however, areemployed for the production of malleableized iron, the latter being softand ductile and of relatively low strength, compared with steel.

It is a matter of common knowledge that gray cast iron, although usedextensively in r the industries, is of relatively low tensile strengthand that it is not possible, although the matrix of gray iron is ofessentially the same composition as steel, appreciably to improve thephysical properties of gray cast iron by the common and well knownhardening and tempering heat treatments that are effectively employedfor the treatment of steel. This is mainly due to the presence of thegraphitic flakes and fissures which in all cases form planes ofweakness. It is, however, possible to improve the physical properties ofgray'cast iron by special methods of treatment. As far as we are awarethe only methods heretofore commercially used, particularly inconnection with the production of sand cast iron are what are known asthe Application filed .Tanuargs 19, 1931. Serial No. 509,879.

superheating method and the mold heating method. In the superheating,the molten iron is heated considerably above the temperatures ordinarilyemployed in the furnace, the object being to increase solubility forcarbonand as a result of such increased solubility the final productcontains its free carbon in the form of smaller graphitic flakes andfissures than is present in the common gray cast iron. Vhile thesuperheating method results in securing an improved iron, this method oftreatment is subject to the objections that the product still containsgraphitic flakes and fissures in an extent sufficient to have aweakening effect, and that the higher tem 'ieratures necessary involvegreater product-ion costs because of the insuch matrix as graphiticflakes. This method is subject to the objection noted above with respectto the formation of graphitic flakes and fissures and is also subject tothe objection of increased production costs and diliiculties in handlingthe heated molds.

We aim by the present invention to T0- duce as a final product an ironfree rom graphitic flakes and fissures with the free carbon in the formof temper carbon and with the remainder of the carbon in a statecorresponding to that found in the transformations, pearlite, sorbite,troosite or martensitc, depending upon the uses for which the product isintended. The present invention may be briefly'said to depend upon apredetermined analysis of the iron, the constituents thereof fallingwithin fairly well defined limits, and, in conjunction with suchanalysis, a heat treatment of the iron after casting so as to controlthe decomposition of the carbon content. The method hereinafter to bemore fully described is particularly applicable to castingin sand moldsalthough'thc invention is not limited to the use of any particular typeof molds:

We have found that an lIOIl, whose constituents fall Within thefollowing limits, when subjected to the heat treatment hereinafter to bedescribed, will have the desired highphysical properties which it is theobject of the present invention to obtain:

Carbon from 1.90 to 2.30% Manganese from .20 to .65% Silicon from 1.50to 2.20% Phosphorus below .12% and Sulphur below .12%.

An iron of the following analysis prepared in accordance with theinvention has given highly successful commercial results:

Carbon 2.08% Manganese 21% Silicon 1.85% Phosphorus 05% and Sulphur 06%.

Molten metal of the above described analysis is cast, preferably in sandmolds, to secu-re an article, or articles, of the predetermined desiredshape and size. Due to the analysis, the casting produced is mainlywhite.

heat treatment will depend upon the cross section of the casting. Forcastings up to one-half inch cross sectional dimensions, a heattreatment at the maximum temperature of one and a half hours durationhas been found to be sufficient.

In some cases the castings are then reheated to a temperature justslightly below the critical range and are allowed to air cool, to insurea matrix which is mainly pearlite (and sorbite) in which is imbeddedfree particles of temper carbon.

The product resulting from the above described method that is with aniron of approximately 2% carbon, is found to have its carbon contentdistributed approximately 1% in the form of a combined carbon, that is,iron carbide free in solid solution in the matrix, or eutectoid andapproximately 1% in the form of free temper carbon interspersedthroughout the matrix. Thisproduct has remarkablyhighphysicalprope'rties, giving on test a'tensile strength of over 90,000

pounds; the hardness of the product is readily controllable as desired.

It will be understood however that the ratios of carbon distribution inthe forms of temper carbon and in solution as iron carbide will varyaccording to the analysis of the batch. It will further be understoodthat such analysis as well as the heat treatment may be varied from thespecific example above described and that such changes, variations andmodifications as may be resorted to without departing from the basicunderlying principles of the invention are intended to be covered by theclaims hereunto appended.

We claim:

1. The method of manufacturing cast iron which consists of preparing abatch of molten metal in which the constituents will be within thefollowing limits:

Carbon from 1.90 to 2.30%. Manganese from .20 to .65% Silicon from 1.50to 2.20% Phosphorus below 12% and Sulphur below 12%,

casting the molten metal in a mold to secure an article of predeterminedconfiguration, and after the article has cooled reheating toapproximately 17 F. for a predetermined period of time, depending uponcross section and analysis, suflicient to decompose the combined carbon,then lowering the temperature to a point somewhat above the criticalrange, and preventing further decomposition at such point by a rapidcooling, suchas quenching, thereby to obtain a matrix composed of solidsolution, and then finally reheating to a point below the criticalrange, to produce a matrix composed of martensite, troosite, sorbite orpearlite as determined by the reheating temperature.

2. As an article of manufacture, heat treated cast iron having acomposition within the following limits:

Carbon from 1.90 to 2.30% Manganese from .20 to .65% Silicon from 1.50to 2.20% Phosphorus below .12% and Sulphur below .12%,

free from graphitic flakes and fissures in which the carbon content issubstantially

